If you do the Hard level Fill-a-Pix I cannot find any other solution to finishing them other than trial and error. In the instructions is says you should never guess, that there are always clues to determining if any square is filled in or not. But when you get the puzzle about 2/3 filled in I can find no other solution than to take a 50/50 square and try it out either filled in or not. Have others found that to be the case or am I just not seeing the clues?

You shouldn't have to guess to solve the puzzle. If you get stuck, put it away for a day. Sometimes looking at it with fresh eyes will help you see something you missed. And you can always post a screenshot in the forums and ask for help.

Yes, there's always a clue. I used to "trial & error" and just knew it's not the way to go. Then one day my eyes opened.

Check out the logical starting points document here, as well as a document I wrote 10 years ago regarding pairs of numbers here. Ignore page one, as well as the email address in the document. Like I said, I wrote it 10 years ago.

These help when you get stuck, but sometimes the puzzles are really, really difficult. I've been staring at a 100x60 (ID: 03290100059) for a long time (years, although it says 34 hours 23 minutes) now, and I'm nowhere near done. Yet, I refuse to "trial & error" because I know there's a clue in there somewhere.